Energy storage devices such as batteries with high energy density and power density, long cycle life and calendar life, good safety and low cost are in high demand to supply power for electronic devices, electric vehicles (EVs) and smart grid. To date, lithium (Li)-ion batteries have been one of the most widely used energy storage systems for portable electronics and EVs. However, the high cost of Li-ion batteries has hindered their adoption for large-scale electric energy storage applications. The large-scale deployment of inexpensive energy storage systems will be transformational for the electric grid by enabling a much larger fraction of electrical energy production from renewable (intermittent) sources. Room temperature stationary Na-based batteries have attracted attention for the applications of renewable energy and the smart grid because of their potential low cost and the abundant sodium resources.
Essential criteria required for large-scale electric utility grid systems and/or large battery packs for vehicles include low cost, high safety, high energy density/efficiency and long cycle life. Na-based batteries offer a highly enticing option as such batteries may be based upon low cost, Earth-abundant materials, available from the oceans, salt brines, and geological deposits. For example, the current price for lithium carbonate is about $5000 per ton. In contrast, the current price for sodium carbonate is only about $150 per ton, almost 35 times lower than the lithium counterpart. In addition, Na-based batteries can take advantage of a favorable redox potential (E0(Na/Na+)=−2.71 V vs. the standard hydrogen electrode, only 0.3 V above that of E0(Li/Li+)) and similar intercalation chemistry to Li+ cations (H. Pan el al. Energy & Environmental Science, 2013, 6, 2338-2360; V. Palomares el al. Energy & Environmental Science, 2013, 6, 2312-2337).
To date, commercial Na metal batteries employ high temperatures (>250° C.) for Na—S (sulfur) or ZEBRA (Na—NiCl2) batteries in which the Na metal is a molten liquid and a solid ceramic electrolyte (e.g., β″-alumina) is used to protect the highly reactive Na metal. The operation at high temperature, however, complicates the materials selection, manufacturing, operating system and hazards associated with large batteries. There has been significant interest in recent years in Na-ion batteries in which Na+ cations are intercalated into hard carbon or form alloys (similar to Li-ion batteries) at the anode and thus are not reduced to Na metal, but the limited capacity and low rate (i.e., power) of these anodes significantly limits the energy density obtainable from these Na-ion batteries. The specific capacity is reduced due to the need to stabilize Na+ cations with carbon, with alloying metals/phosphorus, or within metal oxides and sulfides. In addition, the working voltage of such anodes is often well above that of the Na/Na+ couple resulting in a significantly lower battery voltage (than for Li-ion batteries) and thus a much lower energy storage capability when the anode is paired with a cathode in a battery cell.
Thus, it is ideal to use Na metal as an anode to maximize the energy density and power capability of Na batteries. However, tremendous difficulties have been encountered by battery researchers/developers when utilizing this anode due to the instability of Na metal in conventional electrolytes, which has limited the plating/stripping coulombic efficiency (CE) resulting in poor cycling characteristics due to rapid capacity fading.